A London cop was shot dead in his precinct early Friday while booking in a suspect — after officers failed to find his gun during an earlier search, according to reports.

The custody sergeant was shot dead at about 2:15 a.m. while booking a 23-year-old man into custody in Croydon, the Metropolitan Police said.

The suspect then turned the gun on himself and on Friday was said to be in critical condition in a south London hospital. “No police firearms were discharged during the incident,” Scotland Yard said in a statement.

The suspect was allegedly taken in for having ammunition — but the gun was not found during an earlier search, according to the Sun.

Scotland Yard said it had launched a murder investigation over the dead officer, who was not named as they were “in the process of informing all of the officer’s family.”

“We have lost not only a good skipper but also a real gentleman. One of the best,” one colleague, Stuart James, tweeted. “The unimaginable happened to our police family … RIP brother.”

The Met’s commissioner, Cressida Dick, called it a “truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances.”

“We are currently supporting his family and also have a dedicated team providing support to the officers and those in the custody center who witnessed the shooting,” the top cop said.

Forensic officers at the scene at Croydon Custody Centre
Forensic officers at the scene at Croydon Custody CentreAP

“When a colleague dies in the line of duty the shockwaves and sadness reverberates throughout the Met and our communities,” Dick said.

The force was “in the early stages of the investigation” and “still working to establish the circumstances surrounding the incident,” the commissioner said.

Prime Minister Boris Johnson sent his “deepest condolences” to the murdered cop’s “family, friends and colleagues.”

“We owe a huge debt to those who risk their own lives to keep us safe,” he tweeted.

Home Secretary Priti Patel called it “a sad day for our country and another terrible reminder of how our police officers put themselves in danger each and every day to keep the rest of us safe.”

London’s mayor, Sadiq Khan, said he was “devastated” that a “brave officer” has “paid the ultimate price for helping to keep Londoners safe.”